IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005
3325
Spin and Orbital Moments of Ultra-Thin Fe Films
on Various Semiconductor Surfaces
Jill S. Claydon1 , Daxin Niu1 , Yongbing Xu2 , Neil D. Telling3 , Ian W. Kirkman4 , and Gerrit van der Laan3
The Spintronics Laboratory, Department of Electronics, University of York, York YO10 5DD, U.K.
The Department of Electronics, University of York, York YO10 5DD, U.K.
Magnetic Spectroscopy Group of Daresbury Laboratory, Warrington WA4 4AD, U.K.
Station Scientist of Station 1.1 Daresbury Laboratory, Warrington WA4 4AD, U.K.
The magnetic moments of ultrathin Fe films on three different III–V semiconductor substrates, namely GaAs, InAs and In0 2 Ga0 8 As
have been measured with X-ray magnetic circular dichroism at room temperature to assess their relative merits as combinations suitable
for next-generation spintronic devices. The results revealed rather similar spin moments and orbital moments for the three systems,
suggesting the relationship between film and semiconductor lattice parameters to be less critical to magnetic moments than magnetic
anisotropy.
Index Terms—Fe/InGaAs, interface magnetism, spin electronics, ultrathin films.
I. INTRODUCTION
II. SAMPLE FABRICATION AND MEASUREMENT
EXT-GENERATION spintronic devices are expected to
revolutionize conventional electronic circuitry by utilizing
both the charge and spin of the electron while retaining links
with present day semiconductor technologies [1]–[3]. However,
the development of such devices is at present limited by our abilities in transporting spin into nonmagnetic media [4], [5]. And
so research into breaching the gap in characteristics between
the magnetic materials and semiconductor technologies has become a hot topic of debate. Several technologies have been put
forward to overcome this barrier such as magnetic semiconductors [5], where great advances have brought Curie temperatures
up to 173 K [6] and methods based on ferromagnet/semiconductor contacts [5], including epitaxially grown ferromagnetic
metal contacts, which appear to offer the promise of room temperature spin transport.
After many years of investigation [7]–[11], it has recently
been shown that ferromagnetic films have the capability to retain
magnetic properties to interface thicknesses [12]. However, with
predicted levels of spin transport still to be realized [4], many
questions still remain unanswered about the degree to which interface magnetism and hence spin transport relates to the stability of growth and hence to the lattice match and surface of
the materials involved.
In order to determine whether the property of lattice match
and related film strain might be important for magnetic behavior
close to the interface we have undertaken room temperature
X-ray magnetic circular dichroism (XMCD) investigations into
the properties of ultrathin films grown on substrates of varied
lattice constant. The substrates chosen for study, GaAs, InAs,
and In Ga As were chosen for their lattice constants which,
in a 2:1 relationship with Fe give mismatches of 1.56%,
5.32%, and 0%, respectively.
The specialist substrates used to establish the different lattice
matches were constructed by the EPSRC’s UK National Centre
for III–V Semiconductor Technologies in Sheffield and took the
form of GaAs (100) wafers topped with a 0.3 m epilayer of
GaAs, InAs or InGaAs followed by an As capping layer. The
As capping layer allowed for safe transit between systems and
removed the requirement for pre-growth wet etching usually required to achieve the reconstructed surfaces.
Once in our own MBE system all substrates were initially
heated to 350 C to desorb the As capping layer and then further
annealed for 1 h to 560 C for GaAs, 525 C for InGaAs and
514 C for InAs RHEED analysis of the surface indicated an
As rich 2 4 reconstruction was achieved for GaAs and InAs
while a 1 1 surface was observed for In Ga As. Following
cooling to room temperature 7 ML Fe, chosen for the large
uniaxial anisotropy displayed at this thickness for the Fe/GaAs
system [15], was deposited at a rate of 0.66 ML per minute, followed by the 15 ML Cr cap to protect against oxidation. All
stages of growth were monitored by RHEED measurement and
the samples’ uniaxial magnetic qualities were confirmed ex situ
with longitudinal MOKE measurements.
XMCD measurements were conducted at station 1.1 of the
Synchrotron Radiation Source at Daresbury Laboratory (U.K.)
which was equipped with a flipper magnet with a maximum
field of 0.6 T. This configuration allows illumination of the samples with 85% circularly polarized X-rays parallel/antiparallel
to an applied magnetic field and attainment of XAS spectra measured in these opposite applied field conditions,
and . The
flipper magnet offers the ability to reverse the magnetization
at each measurement point and thus reduces anomalies in the
XMCD or difference spectra. Measurements were made with
the samples at an angle of 45 to the incident X-rays to maximize the effect of both the incident light and the applied field.
N
III. RESULTS AND DISCUSSION
Digital Object Identifier 10.1109/TMAG.2005.855201
During growth RHEED patterns were obtained both prior and
subsequent to the deposition of Fe on the reconstructed semi0018-9464/$20.00 © 2005 IEEE
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005
Fig. 1. RHEED images collected before (i, ii, iii) and after (iv, v, vi) deposition
of 7 ML Fe onto the respective reconstructed GaAs, InAs, and In Ga As
surfaces.
Fig. 3. XMCD spectra for Fe L edge studies of 7 ML Fe on GaAs (i), InAs
(ii) and In Ga As (iii). Solid lines correspond to XMCD spectra, while
dashed and dotted lines represent the XAS spectra taken for opposite applied
fields I and I spectra, respectively.
Fig. 2. MOKE loops taken along, the easy (i, iv, and v) and hard (ii, iii, and
vi) axes of the Fe/GaAs, Fe/InAs, and Fe/In Ga As samples respectively,
show uniaxial anisotropy in all samples. Note the 90 rotation of hard and easy
axes in the case of Fe/InAs.
conductor surfaces. The images, which are shown in Fig. 1,
confirm the epitaxial growth of bcc Fe on the three different
surfaces with the same epitaxial relationship of Fe(100)<001>
In Ga
As
, where
.
Ex situ longitudinal MOKE measurements were also made,
shown in Fig. 2. Although the hard axis of the Fe/GaAs and
the Fe/In Ga As films could not be saturated, the hysteresis
loops clearly show the films to be uniaxial in nature. An interesting aspect of these results is the confirmation of the reversal of easy axis and hard axis directions between the GaAs
and InAs substrates. This result is in good agreement with previous studies [14] and is thought to be indicative of the influence
of the magneto-elastic effect in the establishment of the uniaxial anisotropy. The magneto-elastic effect occurs as a direct
result of the compression or expansion of the Fe film in order
to fit the GaAs or InAs lattice respectively. However, in these
results we can also see that the perfectly lattice matched system
Fe/In Ga As also displays uniaxial characteristics. While
this can not be explained by magneto-elastically induced uniaxial anisotropy the result would seem to support the “chemical”
effect proposed by Kneedler et al. [9] in which the uniaxial magnetic anisotropy (UMA) is derived from the unidirectional nature of Fe-As or Fe-Ga bonds. The results show that both chemical and magneto-elastic effects are in some part responsible for
the observed UMA in Fe/III-V semiconductor heterostructures.
The obtained XMCD spectra, shown in Fig. 3, were analyzed
through application of the sum rules, see (1)–(2), with the values
, and derived from the integrals of the
of the parameters
XMCD and XAS spectra in the manner of Chen et al. [15]. The
number of holes, , was taken as 3.39 for Fe [16] and the degree
of polarization, , was equal to 0.85
(1)
(2)
The values of the spin and orbital moments found from sum
rule analysis and those of the bulk values are described in
Table I. By far the most striking aspect of the XMCD results
is, within error, the similarity of observed magnetic moments
between the three different substrates.
The highest spin moment of all the samples tested is seen
for Fe/GaAs, however, this represents only 72% of the bulk
value. This reduction compared with previous results on Ga rich
CLAYDON et al.: SPIN AND ORBITAL MOMENTS OF ULTRA-THIN FE FILMS
3327
REFERENCES
TABLE I
SPIN AND ORBITAL MOMENTS OF BULK Fe IN COMPARISON TO 7 ML Fe
FILM ON GaAs, InAs, AND In Ga As
4 6 surface [17], [18] may originate from the different nature of the interface for these samples, which were grown on As
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of the bulk value, respectively.
In contrast to the reduction in spin moment the orbital mo/
, for all samples shows a
ment, and hence the ratio
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a perfect lattice match.
IV. CONCLUSION
The spin and orbital moments and hysteresis loops of 7
ML Fe films grown on specially fabricated GaAs, InAs and
In Ga As epilayers have been measured using XMCD and
MOKE, respectively. The results show that magneto-elastic
effects provoked by the difference in substrate lattice constant
have little effect on the spin and orbital moments which are
found to be within error of one another for all substrates. The
tolerance of the Fe films to withstand such magneto-elastic
effects might further suggest Fe/III–V semiconductor heterostructures could, in future provide suitable solutions for
spin-transport devices and hints that the flexibility of these
systems may offer practical manufacturing solutions in the field
of future spintronics device design.
ACKNOWLEDGMENT
This work was supported in part by an EPSRC Advanced
Fellowship and a CASE studentship awarded by Daresbury
Laboratory.
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